The present invention relates to methods of forming rivet joints.
Rivets and methods of joining metal sheets together employing rivets have been known for many years, and involve inserting a rivet into aligned holes drilled through the sheets followed by deforming the rivet by compression so that it expands radially on either side of the sheets and holds them together. Where access to only one side of the sheets is easily available, it is usual to employ a blind rivet for forming the joint. A typical blind rivet comprises a sleeve formed from the rivet material that is located about a mandrel having an enlarged head at one end. Usually the mandrel extends some distance beyond the other end of the rivet sleeve as a pulling stem, and a radially extending flange is provided on the end of the rivet sleeve opposite to the mandrel head. In addition, a point of weakness called a breakneck may be provided in the mandrel in the region of the rivet sleeve. The blind rivet can be inserted through the hole formed in the sheets to be joined beginning with the head of the mandrel until the flange on the rivet sleeve abuts the surface of the sheet on the working side thereof so that only the head of the mandrel and part of the rivet sleeve are located on the blind side of the sheets. The rivet can then be set by means of a rivet setting tool which grasps the pulling stem of the rivet and pulls it away from the proximal end of the rivet sleeve while applying a reaction force to the flanged end of the sleeve. This causes the head of the mandrel to be drawn toward the sleeve and the sleeve to be compressed between the head of the mandrel and the rivet setting tool. The setting tool will apply a sufficient force to deform the rivet sleeve axially, and therefore radially, so that the set rivet will hold the sheets together. After setting of the rivet, the mandrel may be snapped in two at the breakneck and the remainder of the pulling stem can be discarded. The head of the mandrel and any part adjacent to it may be removed and discarded so that only the deformed rivet sleeve remains, although in some circumstances it may be left in the sleeve. Thus, in order to grip the sheets and hold them together, some volume of rivet sleeve material is required to remain on each outwardly directed surface of the metal sheets, this being provided by the flange on one side of the sheets.
This form of riveting has been considered for joining parts of the chassis or housing of an electronics assembly. Such a chassis or housing that has been employed is in the form of a container formed from sheet metal such as pressed aluminium zinc plated steel and having a number of internal, generally rectangular areas for receiving various components, for instances power modules, printed circuit boards, disc, tape and CD-ROM drives etc. Space is normally at a premium in such assemblies and various components will usually be inserted by sliding rectangular modules into the appropriate recesses with no more than one or two millimetres of tolerance. It is not economically feasible to join the various sheets forming the internal and external walls of the housing by spot welding because this would require re-plating of the steel sheets after welding. Also, it is not possible to employ usual rivets in such an assembly because this would require access to both sides of the sheets to be joined by a relatively large xe2x80x9clxe2x80x9d shaped anvil of a setting tool.
Accordingly it has been considered to employ blind rivets for joining the various metal sheets forming the chassis of the equipment. However the formation of protuberances on both surfaces of the sheets to be joined by the rivets is not compatible with the requirement to be able to slide the various components into and out of the chassis, nor is it compatible with the proximity of various joined metal sheets forming internal partitions or bulkheads to other parts of the chassis.
According to one aspect, the present invention provides a method of joining a plurality of sheets by means of a blind rivet, which comprises:
(i) forming holes in the sheets and placing the sheets together so that the holes are in register and form a single hole therethrough;
(ii) inserting a blind rivet into the hole formed in the sheets from a working side thereof, the blind rivet comprising a sleeve positioned about a mandrel that has a head; and
(iii) setting the rivet;
wherein the hole is radially enlarged at the outwardly facing surface of at least the sheet on the blind side, and the sleeve of the rivet is deformed during setting to form a rivet joint in which no part of the rivet is proud of the outwardly facing surface of the sheets at least on the blind side of the sheets.
Thus, the method according to the invention has the advantage that access is only necessary to one side of the metal sheets to be joined (the working side) but that it is possible to form a rivet joint that does not protrude on the other surface of the sheets that are joined (the blind side).
It is possible for the hole to be radially enlarged at the outwardly facing surfaces of both sheets and, after setting of the rivet, for no part of the rivet to be proud of the outwardly facing surface of either sheet.
The rivet sleeve may be shaped to engage the radial enlargement on the working side of the hole, so that it performs a similar function to the flange on the sleeve of conventional blind rivets but does not extend beyond the plane of the outwardly facing surface on the working side. This may be achieved by forming the sleeve with an enlarged part, for example a part that tapers outwards in the form of a conical frustum (referred to herein as xe2x80x9cfrusto-conicalxe2x80x9d) and which engages the radial enlargement in the hole when the rivet is inserted into the hole.
Such a form of rivet is novel per se, and so, according to another aspect, the invention provides a blind rivet which comprises a hollow sleeve having an internal bore, and a mandrel that extends through the bore of the sleeve and has a head for supplying force to the sleeve during setting of the rivet, wherein the sleeve has a substantially frusto-conical portion at its end opposite to the head of the mandrel that tapers radially outwards in a direction away from the head of the mandrel.
The radial enlargement on either side of the sheets may have any appropriate shape and need not necessarily be frusto-conical: it could, for example, be a generally cylindrical counterbore of larger diameter than the hole so that the rivet sleeve would have one part of smaller diameter that could extend through the hole and a second cylindrical part of larger diameter. The invention will be described hereafter with reference to countersinking but it should be appreciated that counterbores or other enlargements may be employed.
Whatever the shape of the part of the rivet sleeve that engages the countersink in the hole, the end surface of the rivet sleeve directed away from the mandrel head is preferably flat so that, when the rivet is inserted into the hole, the end of the sleeve on the working side is substantially flush with the surface of the sheet. It is possible if desired, and especially if manufacturing tolerances require, for the part of the rivet sleeve that engages the countersink to be slightly shorter than the depth of the countersink so that the rivet forms a slight depression in the joined metal sheets.
In an alternative form of rivet, the rivet sleeve may be substantially cylindrical, and will stand proud of the surface of the sheet on the working side when the rivet is inserted into the hole, and is deformed axially and radially into the countersink during setting of the rivet. When the rivet sleeve has such a configuration, it should be appreciated that the sleeve itself has no means of ensuring correct axial location within the hole, and so this must be provided by the setting tool. In addition, the setting tool should have a jaw that bears on the rivet sleeve itself and applies force on the rivet sleeve along the axis of the mandrel to deform the sleeve into the countersink.
The head of the mandrel may have any of a number of forms. In one form the mandrel may have a region at the base of the head that slopes with respect to the axis of the mandrel. In other words the head may be joined to the rest of the mandrel by a frusto-conical portion. With this configuration, when the rivet is set the head of the mandrel applies a radially outward force on the sleeve in addition to an axial force and thereby causes the sleeve to expand radially into the countersink on the blind side of the metal sheets. The shape of the mandrel head in this case will depend on whether or not the mandrel is discarded after setting the rivet. For example, the mandrel head may have a generally flat end surface so that it can be flush with the surface of the sheet on the blind side, in which case it may remain in the rivet after setting if desired or it can be removed.
Alternatively, the mandrel head may have a generally radially extending shoulder portion or shoulder where the head meets the shank of the mandrel so that the rivet sleeve is caused to deform axially during setting of the rivet. Such a mandrel will necessarily have a domed head which, although could be flush with the surface of the sheets on the blind side, will normally remain proud of the surface and so should be removed after setting of the rivet.
According to a further aspect, the invention provides a rivet joint that is formed between a plurality of sheets that have been placed together and have a hole extending therethrough, wherein the hole is radially enlarged at the outwardly directed surface of the at least one sheet and the rivet joint is formed by means of a rivet sleeve that extends into the or each radial enlargement but does not protrude above the outwardly directed surfaces of the sheet at the radial enlargement of the hole.